We recently isolated the human l(3)mbt gene by exon trapping based on its location on the long arm of chromosome 20 (20q), within a region frequently deleted in hematologic malignancies. The human l(3)mbt protein is homologous to the Drosophila lethal malignant brain tumor [l(3)mbt] protein, and both belong to the polycomb group (PcG) family of transcriptional regulatory proteins; PcG proteins play vital roles in the maintenance of repression of key genes such as homeobox-containing and cell cycle regulatory genes. Abnormalities in the expression or function of PcG genes have been shown to play a key role in the development or progression of lymphomas and other cancers. To more completely define the role of the human l(3)mbt gene in hematologic cancers, we propose to define the biologic activities and mechanism of action of l(3)mbt. To characterize the biological effects of l(3)mbt we will express wild type and mutant forms of l(3)mbt in hematopoietic cells, and in Ras transformed NIH 3T3 cells. We will also assess how the lack of l(3)mbt affects hematopoiesis by generating l(3)mbt conditional knock-out mice and analyzing their phenotype. We will define its mechanism of action by identifying l(3)mbt interacting proteins and the multi-protein complex that contains the l(3)mbt protein, and by identifying true 1(3) mbt target genes in myeloid cells, using microarray technology. We will also conduct a careful structure-function analysis of the distinct domains in the l(3)mbt protein to assess their role in its transcriptional regulatory properties, its associated biochemical activities and its biological effects. The PcG family of proteins is being increasingly implicated in carcinogenesis, and these studies will provide insights into the role that l(3)mbt plays in hematologic malignancies.